What Is Activity Monitor (or How to Take Your Mac's Pulse)
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For more detail on the hard disk's role in everything, the Disk Activity tab is the place to look. Here you can see exactly how much data is being, or has been, swapped in and out of virtual memory.

The Disk Usage tab is simple enough. It shows you how much of your hard disk space is being used, and how much is free. If the Space Free section of the pie chart is getting worryingly narrow, you probably need to archive some files off the hard disk, prune your music and movie collections, and maybe consider buying another disk to free up more space. Don't forget that the computer depends on the hard disk for virtual memory; if it starts getting too full, the whole system will slow down.

Finally, the Network tab displays a graph of the amount of data going in and out of your machine over the network. Next time you're downloading some huge .torrent file from somewhere, you should see it show up clearly as a spike on this graph.

What Can I Use All of This For?

The most useful things you can do with Activity Monitor are (1) keep a close eye on your computer's performance, and (2) zap applications or processes that have frozen or crashed.

If performance matters, and you like to know what kind of strain the processor is under at all times, you can tell Activity Monitor to display various indicators for you. Under the Window menu you'll find three different CPU usage displays which you can have floating constantly on your desktop if you wish.

What's more, the humble Activity Monitor icon in the Dock can be set to display any of these graphs, or the network activity or memory usage pie chart. Just go to View -> Dock Icon, and select the process you want to monitor on the Dock.

Even if the processor and memory activity don't interest you, the ability to see what kind of network traffic is coming in and out of your machine might be useful, especially if you're still on a dialup connection.

And if you don't even care about network usage, or indeed any other performance indicator, Activity Monitor can still be a useful tool for finding problem apps and force-quitting them.

Tiger users who never paid much attention to Activity Monitor before might want to revisit it for keeping an eye on their Dashboard Widgets. More than once I've spotted a Widget that I don't use much, gobbling up loads of CPU. I can then take the Widget off my active Dashboard until I actually need it.

Much of what Activity Monitor does can also be done with top, a Unix program run in Terminal. If you're happy using command-line apps, top is probably a quicker way to spot the process IDs of misbehaving apps, which can then be stopped using the kill command. But if the command line puts you off, Activity Monitor's friendlier approach might be a better bet.